Lithography is a process for producing patterns of two dimensional shapes, consisting of drawing primitives such as lines and pixels within a layer of material, such as, for example, a resist coated on a semiconductor device. Conventional photolithography (also called optical lithography) is running into problems as the feature size is reduced, e.g., below 45 nm. Thermo-optical lithography (or tOL) is an example of optical lithography technique, known per se. These problems arise from fundamental issues such as sources for the low wavelength of light, photoresist collapse, lens system quality for low wavelength light and masks cost. To overcome these issues, alternative approaches are required.
Examples of such alternative approaches are known in the field of the so-called nanolithography, which can be seen as high resolution patterning of surfaces. Nanolithography refers to fabrication techniques of nanometer-scale structures, including patterns having one dimension typically sizing up to about 100 nm (hence partly overlapping with photolithography). Beyond the conventional photolithography, they further include such techniques as charged-particle lithography (ion- or electron-beams), nanoimprint lithography and its variants, and scanning probe lithography (or SPL). SPL can be used for patterning at the nanometer-scale.
In general, SPL is used to denote lithographic methods where a probe tip is moved across a surface to form a pattern. Scanning probe lithography makes use of scanning probe microscopy (SPM) techniques. SPM techniques rely on scanning a probe, e.g., a sharp tip, in close proximity with a sample surface whilst controlling interactions between the probe and the surface. A confirming image of the sample surface can afterwards be obtained, typically using the same scanning probe in a raster scan of the sample. In the raster scan the probe-surface interaction is recorded as a function of position and images are produced as a two-dimensional grid of data points.
The lateral resolution achieved with SPM varies with the underlying technique: atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute scanning motions with a precision and accuracy, at any desired length scale up to better than the atomic scale. The two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). In the following, acronyms STM/AFM can refer to either the microscopy technique or to the microscope itself.
In particular, the AFM is a device in which the topography of a sample is modified or sensed by a probe mounted on the end of a cantilever. As the sample is scanned, interactions between the probe and the sample surface cause pivotal deflection of the cantilever. The topography of the sample can thus be determined by detecting this deflection of the probe. Yet, by controlling the deflection of the cantilever or the physical properties of the probe, the surface topography can be modified to produce a pattern on the sample.
Following this idea, in a SPL device, a probe is raster scanned across a functional surface and brought to locally interact with the functional material. By this interaction, material on the surface is removed or changed. In this respect, one can distinguish amongst:
Constructive probe lithography, where patterning is carried out by transferring chemical species to the surface; and
Destructive probe lithography, which consists of physically and/or chemically deforming a substrate's surface by providing energy (mechanical, thermal, photonic, ionic, electronic, X-ray, etc.).
Thermal scanning probe lithography (or tSPL) is an example of SPL method, also known per se. tSPL is a thermo-mechanical lithography method capable of fabricating nano-structures quickly (see Pires et al., 2010, and Paul et al., 2011, cited below). Examples of tSPL methods are described in Knoll et al., 2010 and Paul et al., 2012.
High resolution patterning of surfaces is relevant to several areas of technology, such as alternatives to optical lithography, patterning for rapid prototyping, direct functionalization of surfaces, mask production for optical and imprint lithography, and data storage.
Now, it can be realized that lithographic patterns often are composed of multiscale objects (e.g., patterns). As it can be realized too, such objects are difficult to write efficiently using direct write methods, as the latter are actually optimized for writing the smallest scale features. This issue can be addressed in electron beam lithography by beam shaping. However, beam shaping typically uses rectangular beams, which deteriorates the highest achievable resolution, for which a Gaussian beam shape is required. Correspondingly, two types of instruments are commercially available. In the context of tSPL, high resolution patterning can be achieved by using sharp probe tips. In such a context, it can be realized that the writing of extensive structures is time-consuming, as it requires repeated scanning of closely spaced lines. This problem can be alleviated by “multiplexing” the probe tips, i.e., using different probe shapes and sizes, as suggested by beam shaping in e-beam lithography. However, present inventors have realized that the patterning speed is still limited by the mechanical actuation of the probes. Furthermore, tip-wear can potentially cause additional problems if large scale patterns are to be written.
There is accordingly a need for efficient multiscale patterning methods and apparatuses.